The invention relates to a warp tension controller for a loom, with means for comparing a measuring quantity which is related to the warp tension and a reference quantity, which through an operational amplifier connected as a PI controller, with a feedback circuit comprising a resistor in parallel to an integrating capacitor, controls an adjusting means for the warp tension.
Since in a weaving loom, the warp tension varies not only at short term by the shaft movements, but also varies at long term by the nature of the warp yarn which is handled, the decreasing diameter of the warp beam and other circumstances, whereas for a good quality of the fabric, the warp tension must be maintained as constant as possible, modern looms make use of automatic warp tension controllers, comprising a servo loop in which a sensor for the warp tension, a reference adjusting means, a comparing means for the output signals of these two, an operational amplifier and a control means are connected. The control means usually affects the drive speed of the warp beam, and for obtaining a smooth and stable control, the controller is usually designed as a PI controller, i.e. a controller with proportional as well as integrating action, for which purpose the feedback circuit of the operational amplifier comprises a resistor as well as a capacitor. The integrating part of the operation of the operational amplifier ensures that the warp tension control hardly responds at all to the rapid fluctuations of the warp tension, caused by the shaft movements, which fluctuations are too fast for successful mechanical compensation by the warp tension control.
During operation, the integrating capacitor in the feedback circuit carries a D.C. voltage corresponding to the average output signal of the operational amplifier. If the loom is stopped, an entirely different condition of the warp yarn is obtained mechanically, dependent upon the arbitrary position in which the loom comes to a standstill, so that the D.C. voltage across the integrating capacitor assumes a totally different value than during operation. Nevertheless, as soon as possible, after starting up, the original D.C. voltage across the integrating capacitor should again be reached, in order that the condition of normal operation be regained as soon as possible.
If the yarns on a warp beam are almost used up, that warp beam must be replaced by a new warp beam, after which the yarns of the new warp beam are knotted to the ends of the warp yarns of the woven fabric, and the loom is slowly turned until all knots have passed through the shafts successfully. During this slow turning of the loom, an integrating operation of the warp tension controller is undesirable. Nevertheless, already during the slow turning of the loom, the voltage across the integrating capacitor of the operational amplifier should be brought to the value which will later occur, after a switch-over from slow turning of the loom to the normal operational speed has occurred. Otherwise, at the transition to the norma- operational speed, when the integrating capacitor is again introduced in the feedback circuit of the operational amplifier, a control transient would occur due to the charging of the integrating capacitor.